Ultra-wide-angle lens" that matches the optical performance standards set by the Sigma Contemporary line of lenses.The lens construction consists of two Sld glass elements and three aspherical lens elements. Aspherical lenses are used for both the front and rear optics. This not only keeps the size of the lens compact but also helps with correcting lens aberrations. As a result, the lens features a 55mm filter thread (which is smaller than comparable lenses of similar focal lengths) which makes it easy to change filters between Sigma I series lenses that share the same filter size. Its construction also helps minimize flare and ghosting that is commonly found in ultra-wide-angle lenses, creating clear images in any environment. This is a lens with truly modern performance, thanks to the advanced processing and production technologies at Sigma's only production facility in Aizu.a highlight of using the 17mm is the impressive minimum focusing distance of 12cm.